I. Field of Invention
The present invention relates to a device and method for carrying out cementing operations in a wellbore comprising a casing.
II. Description of Related Art
In the construction of wells, it is a requirement from The Norwegian Oil Directorate that a casing installed inside another casing must be pressure-tight before drilling is performed through the bottom of the last installed casing. During conventional cementing operations, cement is usually injected through a check valve installed at the casing bottom. In order to comply with the pressure requirements, an amount of cement is injected that is sufficient to form a column of at least 50 m height on the outside of and within the casing. The cement is then tested from within the casing using brush plugs, with the check valve at the casing bottom being closed. In order to save time, the casing is tested while the cement is still wet, and if leaks are discovered, additional cement is forced into the leak passage and a new pressure test is performed. Such cement refilling operations are technically challenging and costly, and do not always give a satisfactory result.
In some wells, it is desirable to seat the casing bottom in bedrock having less pressure than shallower rock. The cement discharged through the bottom of the casing will select the path of least resistance, in this case downwardly into the weak zone due to gravity. As a result, the minimum requirement of a cement column extending at least 50 m above bottom level will not be achieved.
In order to obtain a pressure-tight casing, it is common to install a circular valve that is threaded onto the casing 50 m above the bottom level of the casing. Often, a pressure-operated valve is used, in which case a plug is pumped down towards the valve before the cement in order to open the valve, followed by another plug behind the cement for closing the valve. Due to gravity, or driven by an applied pressure, the cement column rises to the required 50 m, so that it may be verified through a pressure test that the casing is in fact pressure-tight. The drawback of this method is that the valve needs to have a wall thickness that causes its outer diameter to exceed the outer diameter of the casing. Moreover, the rotational moment that such a valve is able to support is significantly lower than the moment required for a casing, so that this method is not suitable for applications wherein it is necessary to rotate the casing to “drill” the pipe down to the desired depth. Also, the inner diameter of such a valve is generally less than the inner diameter of the casing, which is a major disadvantage. Furthermore, the seals of these valves have shown to be unreliable, and their pressure rating is less than that for the casings, creating an undesirable weak point in the casing.
Conventional cementing valves also have the disadvantage that the valve mechanism is not isolated from the well liquids. This causes well liquids and possibly cement to penetrate into the movable parts of the valve mechanism, increases the friction, blocks cementing ports, and/or concretes stuck packers, making the valves unreliable. Further, in the conventional technology, no verification is generally obtained at the rig floor of whether or not the cementing valve is functioning properly. The valves are operated by pumping down rubber plugs in front of and behind the cement. The first rubber plug opens the valve by pushing on a sleeve valve. The second rubber plug closes the valve by pumping a sliding sleeve. Due to the complexity of the system and the fact that the work is performed at a depth of several thousand meters using high pumping rates, it is almost impossible to detect a pressure buildup verifying the opening and closing of a cementing valve. In addition, a viscous, compressible oil-based drilling mud is used, with which a delay of several minutes occurs before a pressure buildup can be seen at the rig floor. This may e.g. lead to the incorrect assumption that an adequate amount of cement has been injected into the annulus, when this is not actually the case. Subsequently, this may result in an uncontrolled blowout, which is extremely severe and costly.
In cementing operations, “mechanically operated” cementing valves are frequently used. Such valves may be installed anywhere in a casing and in any number needed in order to seal a well. The valve may be constructed so that its inner diameter equals the inner diameter of the casing and its outer diameter equals the outer diameter of the casing connectors. Currently, the conventional cementing valve design does not exhibit the same pressure rating as the casings do due to a thin wall thickness and a deficient sealing technology. Such a conventional design uses an opening and closing tool, which is used for discharging a pre-selected amount of liquid cement or another liquid through the ports of the cementing valve in order to obtain the desired pressure seal around a casing. In the prior art, the valve is opened and closed through a sleeve seal and valve ports by moving the drill string up and down. When the cementing operation has been completed, the valve is closed and a pressure test of the valve and casing may be carried out. The drill string is disengaged from the cementing valve by rotating the drill string until a tool mounted thereon is no longer locked in locking grooves of the cementing valve. It is also known to use a non-rotational up-down movement in conjunction with a friction lock to open and close the cementing valve, in which case a tool is released from an engagement with a profile of the cementing valve when a given force is applied.
The current conventional solutions suffer from the following drawbacks: The rotational moment is less than that of casing connectors and cannot be verified by calculation. This constitutes a risk in applications wherein “drilling” is performed using the pipe on which the valve is mounted. The worst conceivable scenario is that a valve is split in two parts, so that the casing is severed. The pressure rating of the prior art cementing valves is substantially less than the pressure rating of a casing. None of the prior art solutions exhibits a pre-verifiable calibrated indication on the repeatable opening and closing, or any indication at all of the position in which the individual valve is located or of which valve is actually operated. This makes the operation critical, especially in greatly deviating wells in which, due to vertical and torsional friction, it is difficult to verify the rotation or axial up-down movements at the surface. The lack of verification makes operations critical in that it is a risk of injecting cement to an undesired location, with the worst conceivable scenario being that a drill string is concrete stuck.
Another critical situation that may arise with the prior art solutions is that the valve may be opened in an uncontrolled manner in that equipment unintentionally is run past the valve. The valves are kept closed by frictional forces, that is, only frictional forces from packers and O-rings, which in many cases is not sufficient to prevent the valve from being unintentionally opened. Moreover, the prior art solutions provide no means preventing undesired fluids and solids from entering into the critical parts of the valves, which could easily cause failure of the valve function.
After the cementing job at a given location has been completed and the cementing valve may be closed, it will be desirable to drill further down towards the reservoir by means of the rotatable drill string and associated tools. In addition to that the rotating equipment may help opening the closed cementing valve, the friction between the rotating equipment and the inside of the cementing valve will mill out the inside of the cementing valve so that the material thickness of the inner sleeve of the cementing valve becomes thinner than the original thickness. This impairs the mechanical properties, which could cause the occurrence of leaks. As a worst-case scenario, the impaired mechanical properties could result in a gas leak, which may give rise to a blowout in the well. If the leaky casing cannot be tightened, the well may have to be re-drilled.
The described cementing operations are usually carried out repeatedly, as several is casings are installed within each other in a well, and each time a casing is completed, cementing must be performed. Hence, it is important to have access to equipment that allows the opening and closing operations for the cement mixture to be carried out repeatedly. It is also important that the outer walls of the pipes are level, and it is an absolute precondition that the pipe walls and the cementing valve do not form weak points in the well.
In strongly deviating (non-vertical) wells, gravity will cause the injected cement to fill the bottom of the annulus, and usually no reliable seal is obtained between the pipes in the upper part of the annulus.
U.S. Pat. No. 5,299,640 relates to a cementing device comprising cementing ports that may be opened and closed by way of a sliding valve. The valve may be opened and closed using a drive that is operated by means of suitable received signals.
The Norwegian application 2005 3880, of the same applicant, relates to a cementing valve of the above kind. The device according to the invention is characterized in that the cementing valve may be joined between casing sections, the inner and outer diameters of the cementing valves being substantially equal to the inner and outer diameter, respectively, of the casing, and the mechanical properties of the cementing valve being similar to or having a higher rating than the mechanical properties of the casing. The cementing valve includes an inner sliding sleeve which in a closed position covers a number of openings through an outer pipe surrounding the inner sliding sleeve, and in an open position uncovers said openings. The sliding sleeve includes an actuating means requiring a predetermined force for being actuated both from the closed position to the open position and vice versa, engaging means being arranged on the inside of the sliding sleeve for being engaged by a well running tool comprising corresponding gripping means.